A passive distance measuring apparatus has been known as a conventional distance measuring apparatus for accumulating signal charges from a light-receiving means such as a sensor array and measuring distance in accordance with an output signal representing the signal charges. Some distance measuring apparatus of this type has a charge transfer means such as a CCD as a means for reading the signal charges from the light-receiving means.
An active distance measuring apparatus which projects spot light to an object the distance to which is to be measured and receives light reflected by the object to perform distance measurement by triangulation is also known. Some distance measuring apparatus of this type additionally uses a sensor array such as a CCD as a light-receiving means.
Most of the distance measuring apparatuses having the charge transfer means such as the CCD described above have a function of multiplying an output signal by a gain so as to accurately measure distance regardless of the magnitude of a light reception signal amount. As an implementation example, some apparatus has a gain amplifier in the output stage of a CCD sensor, and some apparatus has an arrangement in which a digital signal obtained by A/D conversion in a CPU for arithmetically operating a signal from a CCD sensor is multiplied by a gain. In a distance measuring apparatus having such an arrangement, the gain of an output is made high to implement high-reliability distance measurement operation when the light reception signal amount is small, while the gain is made low to prevent signal saturation when the light reception signal amount is large.
In an image sensing apparatus including an image forming function in addition to the above-mentioned distance measuring apparatuses, a large amount of dark current is generated along with an increase in temperature. When a high gain is used for a signal from a light-receiving unit, the signal saturates to result in signal processing errors in subsequent operations. In particular, in the distance measuring apparatus having the charge transfer means as described above, a dark current except a light reception signal charge is generated. This dark current abruptly increases with an increase in temperature, as shown in FIG. 11. The dark current almost doubles with an increase in temperature of 10° C. In addition, when the output signal is multiplied by the gain, as described with reference to the above prior art, the dark current is also multiplied by the gain. The dark current has no influence at room temperature, but the output signal may saturate with a high gain at a high temperature. The magnitude of the gain is determined by a dark current generated at the highest temperature guaranteed by the distance measuring apparatus.